Impact copolymers (ICP's) are commonly used in a variety of applications where 10 strength and impact resistance are desired, such as molded and extruded automobile parts, household appliances, luggage, and furniture. Propylene based ICP's are typically an intimate mixture of a continuous phase of a crystalline polypropylene polymer and a dispersed rubbery phase of a secondary polymer, e.g., an ethylene copolymer. While these so-called ICP products have been produced by melt compounding the individual polymer components, 15 multi-reactor technology makes it possible to produce them directly. This is accomplished by polymerizing propylene in a first reactor and transferring the polypropylene polymer from the first reactor into a second reactor where the secondary copolymer is produced in the presence of the polypropylene polymer.
A variety of polymerization processes can be used to prepare the crystalline 20 polypropylene polymer and the secondary polymer, such as gas phase, slurry, liquid, and/or solution polymerization. It is also quite common to make the constituent polymers in two different polymerization processes, for example, slurry phase for the polymerization of the polypropylene polymer and gas phase for the polymerization of the ethylene copolymer.
One problem encountered in a two stage or two step polymerization process that uses gas phase polymerization for the secondary ethylene copolymer is that the molecular weight of the polymer is lower than desired. Another problem is that the amount of the secondary ethylene copolymer produced with respect to the crystalline polypropylene polymer is lower than desired.
There is a need, therefore, for improved polymerization processes for making ICP's having an increased amount of the secondary ethylene copolymer relative to the crystalline polypropylene polymer and an increased molecular weight of the secondary ethylene copolymer.